Various substituted benzimidazoles are known to be useful as herbicides. U.S. Pat. 3,325,271 describes the use of a broad class of substituted benzimidazoles as herbicides in which there is at least one substituent on the aromatic ring or at the 1 or 2 position of the molecule. The 1 and 2-substituents may be alkyl, and the aromatic substituents are selected from nitro, halo, lower alkyl, lower alkoxy and halo-lower alkyl. Other patents relate to specific benzimidazole compounds, especially those having at least one trifluoromethyl substituent, either on the aromatic ring or at the 2-position. Motoichi Itaya, Yakagaku Zasshi, Volume 82, pages 1-5 (1962) describes the synthesis of certain 5(6)-substituted 2-methylbenzimidazoles in which the 5(6)-substituent may be a branched chain alkyl group such as isopropyl, tert-butyl, and isoamyl (see Chem. Abstracts, Volume 57,9840).
This invention relates to a class of specific 1,2,5-substituted benzimidazoles which are useful as selective herbicides. The compounds of this invention possess improved selective herbicidal properties over the closest related compounds of Itaya, especially when used as a pre-emergence treatment.
According to the present invention, there are provided benzimidazole compounds of the formula: ##STR1## in which each of R.sub.1 and R.sub.2 are alkyl of 1 to 3 carbon atoms, R.sub.3 is a branched chain alkyl group of 3 to about 6 carbon atoms, and X represents hydrogen or halogen, especially bromine and chlorine. Also provided are the N-oxide derivatives thereof. Thus, the groups represented by R.sub.1 and R.sub.2 may be methyl, ethyl, n-propyl or isopropyl; preferably R.sub.1 and R.sub.2 are methyl. The branched chain alkyl groups represented by R.sub.3 may be isopropyl, tert-butyl, sec-butyl, tert-pentyl, cyclopropyl, and the like. Preferably, R.sub.3 is isopropyl or tert-butyl, and X represents hydrogen.
The compounds of this invention are generally crystalline solids or oils which are soluble in many common organic solvents such as ethanol, benzene, n-hexane, etc. They will form salts with mineral acids such as hydrochloric acid and some organic acids, such as trifluoroacetic acid, and such salts are generally soluble in water. The compounds may be readily prepared by several procedures such as by reaction of the corresponding ortho-phenylenediamine with an alkanecarboxylic acid or carboximidate. The various intermediates are prepared by synthetic procedures well-known to the organic chemical art. A typical reaction scheme is illustrated below, in which R.sub.1, R.sub.2, R.sub.3 and X have the significance previously assigned. ##STR2## The benzimidazole-forming reaction employs the carboximidate, generally in its hydrochloride form. When an alkanecarboxylic acid is used to form the benzimidazole, the reaction takes place in the presence of a mineral acid. The desired products are isolated from the reaction mixtures and purified by conventional procedures.
The N-oxide derivatives are prepared by the catalytic reductive cyclization of the corresponding 2-nitroacylanilide, such as by hydrogenation over palladium catalyst. The reaction can be illustrated as follows: ##STR3## A small amount of the corresponding o-phenylenediamine is also formed in the reaction, but may be separated by taking advantage of differences in solubility of the products in hydrocarbon solvents.
The following examples illustrate the preparation of representative compounds of this invention.